An optical frequency comb is an optical spectrum of equidistant lines. The use of frequency combs as a tool may depend on the spacing between teeth (lines) of the comb. Mode-locked lasers can provide accurate frequency combs. A mode-locked laser produces a stream of identical pulses, which can have a duration of only several femtoseconds, at a repetition rate typically in the range of 70 to 150 MHz. In the frequency domain, the laser output consists of a frequency comb having equidistant lines. Because these lines are exactly equally spaced, this frequency comb can be used as a ruler for optical frequencies. Frequency combs may provide enhancements in other areas of measurement.